We all know the dangers of not balancing our check books: we could withdraw from our bank accounts more than we’ve deposited, and get fined-or worse-for overdrawing.

You’d think we’d manage our groundwater accounts at least as carefully as our bank accounts, especially given that the food security of this and future generations depends on them. But we don’t. Rarely is groundwater use monitored, measured or regulated. This is true for most of the world, as well as for California’s Central Valley-the fruit and vegetable bowl of the United States. Farmers in the 52,000 square-kilometer valley produce 250 different kinds of crops that together account for 8 percent of the nation’s agricultural value.

But thanks to the National Atmospheric and Space Administration’s (NASA) satellite mission called GRACE (the Gravity Recovery and Climate Experiment), we’re getting some excellent assessments of what’s happening to water underground-and the picture is sobering. GRACE monitors changes in Earth’s gravity field that result from changes in water storage, and the technology can give a fairly accurate picture of what’s happening to groundwater supplies.

Satellite photo of groundwater-irrigated farmland in an arid region of California courtesy of NASA.

Jay Famiglietti of the Center for Hydrologic Modeling at the University of California, Irvine, and eight colleagues used data from the GRACE mission to estimate that California’s Central Valley lost 20.3 cubic kilometers (16.4 million acre feet) of water between October 2003 and March 2010–a volume equal to 63 percent of the capacity of Lake Mead, the nation’s largest reservoir. Most of that depletion occurred between April 2006 and March 2010, a period of drought when farmers pumped more groundwater to compensate for less rainfall and cutbacks in surface water deliveries to irrigators. Famiglietti’s team published their findings this month in Geophysical Research Letters.

A short-term depletion of groundwater isn’t necessarily a bad thing. Similar to the way we dip into a rainy-day fund, it may make sense to deplete reserves during hard times, such as a dry spell. But unless the reserves are replenished, there is less water available to tap during the next drought–or to meet the food demands of the coming generations of Americans. And with population growth and climate change adding to the water stresses of the U.S. West and other parts of the world, replenishing critical groundwater accounts is likely to get harder.

Scientists have also used GRACE to estimate that northwest India, that nation’s breadbasket, lost 109 cubic kilometers (88.3 million acre feet) of groundwater between 2002 and 2008–a volume double the capacity of India’s largest surface reservoir and more than triple the capacity of Lake Mead. The findings for India are particularly alarming because rainfall was slightly above normal during the period studied. Presumably, during drought years the depletion rate would be even higher.

The world’s future food security depends on sustainable and reliable supplies of groundwater. Measuring and monitoring what’s happening to underground water reserves is an important first step. But this must be followed by the adoption of policies and management practices to arrest the depletion and balance the groundwater books.

Sandra Postel is director of the Global Water Policy Project and lead water expert for National Geographic’s Freshwater Initiative. She is the author of several acclaimed books, including the award-winning Last Oasis, a Pew Scholar in Conservation and the Environment, and one of the “Scientific American 50.”

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